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Considerable uncertainty has surrounded the occurrence of tetrahedrally coordinated Al and B at the T site in tourmaline. Although previously detected in several tourmaline specimens, the frequency of these substitutions in nature, as well as the extent to which they occur in the tourmaline structure, is not known. Using 11 B and 27 Al MAS NMR spectroscopy, we have investigated the presence of B and Al at the T site in 50 inclusion-free tourmaline specimens of low transition-metal content and different species (elbaite, "fluor-elbaite," liddicoatite, dravite, uvite, olenite, and magnesiofoitite) from different localities worldwide. Chemical shifts of [4] B and [3] B in 11 B spectra, and [4] Al and [6] Al in 27 Al spectra, are well resolved, allowing detection of even small amounts of T-site constituents. In the observed spectra, [4] B and [3] B peaks are located at 0 and 18-20 ppm, respectively, with the greatest intensity corresponding to [3] B (=3 apfu). In 27 Al spectra, [4] Al and [6] Al bands are located at 68-72 and 0 ppm, respectively, with the greater intensity corresponding to [6] Al. However, inadequate separation of Y Al and Z Al precludes resolution of these two bands. Simulation of 11 B MAS NMR spectra shows that tetrahedrally and trigonally coordinated B can be readily distinguished at 14.1 T and that a [4] B content of 0.0-0.5 apfu is common in tourmaline containing low amounts of paramagnetic species. 27 Al MAS NMR spectra show that Al is also a common constituent of the T site in tourmaline. Determination of [4] Al content by peak-area integration commonly shows values of 0.0-0.5 apfu. Furthermore, the chemical shift of the 27 Al tetrahedral peak is sensitive to local order at the adjacent Y and Z octahedra, where [4] Al-Y Mg 3 and [4] Al-Y (Al,Li) 3 arrangements result in peaks located at ~65 and ~75 ppm, respectively. Both 11 B MAS NMR and 27 Al MAS NMR spectra show peak broadening as a function of transition-metal content (i.e., Mn 2+ + Fe 2+ = 0.01-0.30 apfu) in the host tourmaline. In 11 B spectra, broadening and loss of intensity of the [3] B signal ultimately obscures the signal corresponding to [4] B, increasing the limit of detection of [4] B in tourmaline. Our results clearly show that all combinations of Si, Al, and B: T = (Al, Si) 6 , T = (B, Si) 6 , T = (Al, B, Si) 6 , and T = Si 6 apfu, are common in natural tourmalines.

Tourmalines of unusual (mushroom) habit are common in granitic pegmatites of Momeik, northeast of Mogok, Myanmar. Here, we examine a sample of elbaite of significantly different habit, consisting of a series of diverging crystals, resembling a sheaf of wheat and ranging in colour from light purplish-red at the base to dark purplish-red at the tip with a thin green cap at the termination. The crystal structures of eight crystals are refined to R1-indices of ∼2.5% using graphite-monochromated Mo-Kα X-radiation; the same crystals were analysed by electron microprobe. 11B and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra were collected on four regions of the wheatsheaf crystal, and show ∼0.3 a.p.f.u. [4]B and <0.1 a.p.f.u. [4]Al in the structure. 57Fe Mössbauerspectr oscopy was done on the dark green rim at the termination of the crystal, showing all Fe in this region (∼0.6 a.p.f.u.) to be Fe2+. Detailed electron-microprobe traverses show that the principal compositional variation involves the substitutions [4]B + YAl → Si + YFe*, where transition metals are present, and [4]B2 + YAl → Si2 + YLi, where transition metals are not present, although several other minor substitutions also affect crystal composition. Successive microscopic bifurcation of crystallites causes divergence of growth directions along the c axis, imparting the overall ‘wheatsheaf’ shape to the crystal aggregate. We suggest that such bifurcation is common in pegmatitic elbaite crystals, resulting in their common divergent habit.

Bavenite is an orthorhombic calcium beryllium aluminosilicate, a ≈ 23.2, b ≈ 5.0, c ≈ 19.4 Å, V ≈ 2250 Å3, Z = 4, that crystallizes in the space group Cmcm. The crystal structures of 24 bavenite samples from various localities worldwide were refined to R1 values from 2.4 to 7.5% based on ∼1330 unique reflections collected with Mo-Kα X-radiation on a Bruker P4 CCD single-crystal diffractometer. The composition of each crystal was determined by electron microprobe analysis. There is extensive solid-solution in bavenite according to O(2)OH– + T(4)Si4+ + T(3)Be2+ ⇌ O(2)O2– + T(4)Al3+ + T(3)Si4+, such that the general formula may be written Ca4BexSi9Al4–xO28–x(OH)x, where x ranges from 2.00 to ∼3.00 a.p.f.u. Small amounts of additional Be may be incorporated into bavenite via the substitution T(3)Be + O(2)OH– + Na + T(4)Si2 ⇌ T(3)Si + O(2)O2– + Ca + T(4)Al2. Local (short-range) bond-valence considerations indicate that Short-Range Order (SRO) should be extensive in the bavenite structure, and this is confirmed by Fourier Transform Infrared (FTIR) spectroscopy in the principal OH-stretching region and by 27Al Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) spectroscopy. Short-range bond-valence considerations indicate that the T(3)–T(4)–T(3)–T(4) rings of the framework can incorporate any short-range arrangement of cations consonant with their site populations [T(3) = Be,Si; T(4) = Si,Al], and 27Al MAS NMR spectroscopy confirms this, showing the presence of the local clusters T(3)Be–T(4)Al–T(3)Be, T(3)Si–T(4)Al–T(3)Be and T(3)Si–T(4)Al–T(3)Si. Incorporation of Be at the T(3) site is accompanied by local replacement of O2– by (OH)– at the O(2) site and hydrogen bonding to the adjacent O(3) anion; the latter promotes Be → Si substitution at the T(3) tetrahedron adjacent in the b direction. T(3)–T(4)–T(3)–T(4) rings link in the c direction through a T(3)–(1)–(3) linkage [T(1) = Si]. Local bond-valence considerations show that occupancy of both T(3) tetrahedra by Be violates the valence-sum rule, and that the linkage T(3)Be–T(1)Si–T(3)Si provides the constraint whereby Be does not exceed 3 a.p.f.u. in bavenite when incorporated via the substitution O(2)OH– + T(4)Si4+ + T(3)Be2+ ⇌ O(2)O2– + T(4)Al3+ + T(3)Si4+.

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